What is the best way to unwrap a sphere?
I find the results are best when islands best match the actual shape of the geometry.  The closer you get to this the less stretching you will have.

For a sphere, I find this works best for me.  A cube with sub-D at level 2 which is applied then corrected in Edit mode with the To-Sphere tool. 

Then 

Or even better for less stretching.  (however, more seams mean more chance for the lighting to show artifacts with specular highlights on normal maps).



EDIT:  It just occurred to me that some of the issues which people are having with stretching in their textures may be related to the Sub-Divide UV's  Option in either of the two sub-D modifiers(Multi-res).

If you don't set this button appropriately, then you will likely have stretching and it matter matter how well the UV's are laid out.

As a general guideline, it seems best to disable the Sub-Divide UV's option when your mesh is mathematically evenly spaced.

So when you have built your mesh with somewhat un-evenly spaced lines, then you may want to leave Sub-Divide the UV's on for the least amount of stretching.  

I toggle it on and off rapidly while watching the model and how a checkered texture is become stretched or not.
I do the same thing with the little eyeball icon that turns the modifier on and off.

Keeping an eye on this should help minimize the baking difficulties.